1. Field of the Invention
The present invention relates generally to a control system for controlling operation of an electric motor, particularly for the motor applied for simulating an automotive internal combustion engine for testing an engine driven vehicular component, such as an automatic power transmission, an automatic transaxle, a differential gear unit and so forth. More specifically, the invention relates to a system for monitoring an electric motor being driven at an over-loaded state.
2. Description of the Background Art
An automatic power transmission testing apparatus employing an electric motor has recently been proposed with an engine output characteristics simulating system. Such system is called as a transient-dynamometer (TR-DY), in which a high or a great inertia in the electric motor has been compensated for simulating the automotive engine driving characteristics with satisfactorily high response. In such system, a direct-current motor is employed as a driving torque source. The direct-current motor employed in such system tends to be driven in an over-loaded condition in view of the capacity defined in continuous rating.
In general, the rating of the electric motor is defined by:
(1) it should not become over-loaded for nominal rating torque which is a temporary maximum output;
(2) a 25% ED rating should be standard, which 25% ED means in the cyclic drive in a given pattern that 25% is driven at the nominal rating torque and remaining 75% is driven at a no-load condition; and
(3) a continuous drive is possible when RMS value (square-average) in each cycle is lower than or equal to 50% of the nominal rating torque.
As seen from FIG. 3, assuming that the torque at respective cycles of .DELTA.t.sub.1 to .DELTA.t.sub.3 are respectively T.sub.1 to T.sub.3, the RMS value and the condition (3) can be expressed by: ##EQU1## By dividing both sides by T.sub.0 and setting T.sub.1 /T.sub.0 =T.sub.a and T.sub.2 /T.sub.0 =T.sub.b and T.sub.3 /T.sub.0 =T.sub.c, the foregoing equation (1) can be modified, if T1=t2=T3=T, as ##EQU2##
On the other hand, driving at 25% ED is expressed as an RMS value by the following equation ##EQU3##
From the foregoing it should be appreciated the value representative of RMS value at 25% ED becomes 50% of the nominal rating torque.
For the motor which is expected to be driven in an over-loaded condition, a driving pattern is set so that the magnitude of over-loading cannot be greater than a predetermined limit. In the prior art, a thermal relay or a temperature switch is used for detecting over-heating due to excessive load for protecting the motor from an excessive over-loading condition.
However, none of the conventionally attempted ways for detecting over-loading condition are successful in view of accuracy and response time required for detection of excessive load.